Transcript Stern_F_GSA_11_Oct_2011

Mantle evolution recorded in Crspinel from the Bay of Island
Ophiolitic Complex, northern
Appalachians.
Fabio Stern1, Keiko Hattori1 and Jean Bédard2
1.University of Ottawa, Ottawa.
2.Geological Survey of Canada, Québec.
Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
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Introduction
Cr-spinel (FeCr2O4) is a useful petrogenic indicator of
mafic-ultramafic rocks since it is not easily altered.
Therefore we examined Cr-spinels and olivines from
the ultramafic complex to evaluate the mantle evolution
of the BOIC.
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Location
The area is located on the west coast of Newfoundland.
It is part the western ultramafic belt of the northern
Appalachians.V
The Bay of Islands Ophiolite Complex is
exposed in four massifs:
• Tablelands
• North Arm
• Blow Me Down
• Lewis Hills
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Geological Settings
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Harzburgite
harzburgite
opxnite
1m
50cm
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Harzburgite
0.5mm
Chr
Ol
Chr
Opx
Ol
0.5mm
3mm
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Dunite
Dunite contains lenses and veins of coarse grained wehrlites.
wehrlite
50 cm
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Dunite
Ol
Chr
Chr
Chr
Ol
Ol
0.5mm
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Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
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Results
•
Harzburgites have high Ir-type PGEs (Ir, Os, Ru) typical of residual mantle rocks;
•
Dunites have higher concentration of Pt and Pd, suggesting a cumulate origin.
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Results
Harzburgite
spinel
Chr
All harzburgite samples plot in
the Ol-Sp 0.5mm
mantle array (OSMA)
defined by Shoji Arai (1994).
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel
mantle array. After Arai (1994).
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Results
Dunite
spinel
Dunites with Ol (Fo>90, ) plot in
the OSMA;
0.5mm
Dunites outside the OSMA were
products from fractionated melt.
Not considered for the evaluation
of the mantle.
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel
mantle array. After Arai (1994).
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Results
Hz and Dun
Large variation in the Cr#;
Max
Abyssal
Per.
Change in mantle conditions
from fertile to more refractory
Chr
(Abyssal field to Forearc
peridotite).
0.5mm
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel
mantle array. After Arai (1994).
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Results
Wide range from MORB to Boninitic fields
Chr
0.5mm
Modified from Kamenetsky et al. (2001).
Modified from Dare et al. (2008). Boninites data from Barnes &
Roeder (2001).
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Results
fO2 calc. based on Ballhaus
(1991) Ol-Sp exchange
thermometry.
Asthenospheric
mantle
Results:
Chr
•Hz shows low fO2;
•Dun shows higher fO2,
suggesting the influence
of subduction.
Modified from Wang et al. (2008).
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Subduction initiation
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Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
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Conclusions
• PGE abundance is effective in identifying its origin.
• Dunite in BOIC is a cumulate based on low Ir-type PGEs
• The mantle source had low S;
• The mantle changed gradually from abyssal peridotite-like to more
refractory conditions in subduction setting;
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References
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